1. Field of the Invention
The present invention generally relates to a driving device of a light emitting unit, and more particularly, to a driving device of a light emitting unit in a pixel circuit.
2. Description of Related Art
Along with the development of electronics technology, the demand for visual services on consumable electronic products has been ever increasing. A high image display quality is desired in such an electronic device as a TV set, a personal computer, or a mobile phone, etc. Various flat panel display techniques have been provided, such as the organic light-emitting diode (OLED, also referred to as organic electroluminescence (OEL)). Compared to other flat panel display techniques, the OLED technique has such advantages as low power consumption, ultra thinness, light weight, self luminosity, unlimited viewing angle, instant response, high photoelectric efficiency, backlight structure and color filter structure independence, high contrast, high luminance efficiency, high brightness, capability of fabricating multicolor and RGB components, and wide working temperature range therefore is considered one of the most promising flat panel display techniques.
A flat display panel usually adopts light-emitting diodes (LEDs) or OLEDs as pixel devices thereof, and a driving circuit of the flat display panel is usually composed of a transistor structure referred as 2T1C (i.e., two transistors and one capacitor). A supply voltage and the threshold voltages of the transistors are parameters in the expression of the driving current generated by a 2T 1C driving circuit. Along with the increase in the size of display panel, the circuit for supplying power to the driving circuit is prolonged, and accordingly the equivalent impedance and voltage drop in the circuit are also increased. As a result, each pixel circuit receives a different supply voltage because of different distance of the pixel circuit to the power source, so that different driving currents are generated by the driving circuit. Accordingly, each pixel presents a different brightness so that an uneven brightness is produced on the display panel. Besides, because it is difficult to control each transistor in a large-sized display panel to have the same threshold voltage, the driving circuit having the 2T1C structure generates different driving currents along with different threshold voltages of the transistors. Thereby, how to resolve aforementioned problems has become one of major subjects in the development of large-sized liquid crystal display (LCD) panels.